ADDIS ABABA, Ethiopia — Nine bombs exploded across Ethiopia’s capital Friday, killing four people and wounding dozens in what police said was a coordinated attempt to discredit the government.

No group immediately took responsibility for the blasts, which focused on government-owned companies and public transportation. But the bombings could point to growing militant opposition to the government.

The attacks came days before Monday’s anniversary of last year’s general election. International observers had called the balloting seriously flawed and opposition politicians have refused to take up their posts to protest what they called government rigging.

Police have blamed other small bombings in recent months on alleged militant elements of the opposition. No groups claimed responsibility for those bombings, which rarely caused injuries or significant damage. All the leaders of the main opposition parties are now in prison and standing trial for treason and genocide.

In the past, other groups such as Oromo Liberation Front and the Ogaden National Liberation Front, both fighting for greater autonomy for their regions, have claimed responsibility for attacks outside Addis Ababa. Agents from neighboring Eritrea are also frequently blamed.

“This is just a futile attempt to (give) the impression that there is no peace or tranquility in the city,” federal police spokesman Commander Demsash Hailu said, reading a government statement. “However, it is evident that the people of Addis Ababa, as usual, are leading their lives.”

Demsash said a special investigative team would work day and night to track down those responsible. He declined to discuss suspects.

“This is just one of the situations that reveals the hopelessness of the terrorists who have been relentlessly working to disturb peace and order in the city,” he added. He put the official casualty toll at four dead and 42 wounded.

Two batches of bombsFour bombs exploded in the morning, followed by a lull before five went off after midday. The bombs struck across the city, but four exploded in the busy Mercato, the main shopping district.

The first blast was at 4:50 a.m. in one of Addis Ababa’s main plazas, damaging an Ethiopian Airlines office.

The next blast damaged the headquarters of the city electricity company, injuring seven. Then an explosion blew out the front of a city bus, injuring seven, witnesses said.

Genet Wordofa, sitting in the hospital next to her badly injured adult son, said she was about to get on the bus when it exploded.

“I was shocked,” she said. “All of a sudden my son was on the ground bleeding.”

A bomb then detonated outside a cafe in the Mercato, killing two people and injuring seven, witnesses said. Bereket Betiwibid, 15, described seeing patrons knocked from their chairs and two waitresses thrown across the verandah when the bomb exploded outside.

Bloody sceneTiny holes dotted the walls where bomb fragments appeared to have hit, and the floor was covered with pools of blood.

In the afternoon, a third man was killed when a bomb destroyed the rear of a small bus, an Associated Press reporter at the scene said. Demsash said 16 people were injured in that blast.

A second afternoon blast destroyed a minibus taxi in front of a secondary school, witnesses said. There were no immediate reports of casualties.

Two more bombs detonated near the bus station in the Mercato, one in a public toilet inside a hotel, injuring three, and the second outside a barbershop, injuring two. A ninth bomb later went off under a bridge but caused no injuries or significant damage.

It was not clear which attack had caused the fourth death.

The free press group Reporters Without Borders called Friday on the government to release political prisoners arrested following last year’s election, including 21 journalists charged with treason and genocide. Most of the prisoners were arrested in November and December, when most of the country’s independent newspapers were shut down.

The group called on Prime Minister Meles Zenawi to release the prisoners on Monday under an amnesty program designed to ease political tensions in the country.